The invention relates to a force-measuring cell for a weighing scale, and it also relates to a weighing scale with a force-measuring cell.
Known force-measuring cells of weighing scales are equipped with a force transducer that is on one side attached to a scale housing serving as support base and on the other side to a weighing pan carrier through which the force to be measured is introduced.
As described in [1], DE 199 39 633 A1, the force transducer which is given the names “counterforce” or “force receiver” in [2], EP 0 670 479 A1, has an elastically deformable body connecting a housing-mounted fixed part of the transducer to a force-application part or, in the case of a weighing scale, to a weighing-load application part. The force transducer has transverse grooves at the transitions between the deformable body and the parts that serve to connect the force transducer to the scale housing and the weighing pan carrier. The transverse grooves serve to mechanically uncouple the deformable body in which the deformations effected by the applied forces are measured by means of sensors, preferably by means of strain gauges.
The deformable body can be configured as a parallelogram-shaped measuring element with an arrangement of guide members resembling a parallelogram (see for example [3], EP 0 511 521 A1).
The analog signal representing the measurement is generated by means of strain gauges that are connected to each other in a bridge circuit. The signal is digitized in a converter circuit and subsequently put through further processing steps. The principal structure of a bridge circuit with strain gauges is described, e.g., in [4], U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik, 11th edition, first reprint, Springer Verlag, Berlin 1999, pages 1242–1243.
To support the further processing of the digitized measuring signal, the measuring cell described in [2] has a memory module in which compensation data are stored that are specific to the individual measuring cell and are used for the correction of the measuring signals.
As described in [5], Patent Specification GB 1 462 808, the aforementioned correction applies in particular to errors that are caused by non-linearities, hysteresis phenomena, temperature and creep effects. The compensation data for the correction are determined during production at the factory through specific test and measuring procedures and are stored in the memory module (also see [1]).
However, one way to handle the foregoing measurement anomalies is through an appropriate design of the measuring cell, so that the errors are kept as small as possible and only a small amount of compensation is required. A solution is proposed in the published patent application EP 0 702 220 A1 [6], wherein the four strain gauges of the measuring bridge circuit, adjustment resistors serving to trim the bridge, as well as temperature-dependent resistors serving to correct errors related to temperature deviations of the strain gauges and of the deformable body are integrated in a printed, circuit that is produced through a thin-film deposition process. This printed circuit is intimately joined to the deformable body over a wide area and connected by means of a flexible flat ribbon cable to a circuit arrangement that serves to process the output signal of the bridge circuit (also see [2], FIG. 2).
However, this involves the risk that the printed circuit on the deformable body may have an unfavorable effect on the measuring properties of the deformable body. Consequently, a simpler circuit configuration for the measuring cell is often preferred, where only the strain gauges are arranged on the deformable body, joined into a measuring bridge, and connected to a circuit arrangement by means of connecting leads.